Methods for operating an internal combustion engine having an exhaust-gas turbocharger are already known. For this purpose, an actuator for influencing the charge air pressure generated by a compressor of the exhaust-gas turbocharger is triggered. This actuator is normally a bypass valve in a bypass, which bypasses the turbine of the exhaust-gas turbocharger or the compressor of the exhaust-gas turbocharger. The actuator, however, may also be an actuator for varying the geometry of the turbine of the exhaust-gas turbocharger.
Exhaust-gas turbochargers having such a variable turbine geometry are currently widely used to achieve a significant increase in the performance of the internal combustion engine. Compared to conventional exhaust-gas turbochargers featuring a bypass valve, the variable turbine geometry particularly allows for an improvement in response in the event of a positive load change, i.e. a load change in the direction of a higher load as occurs in a starting process for example.
Efforts are made in engine development to apply this advantage compared to a conventional exhaust-gas turbocharger having a bypass valve to spark-ignition engines as well. No exhaust-gas turbochargers having a variable turbine geometry are,as yet in mass production.
Due to their design, the currently used exhaust-gas turbochargers featuring bypass valves generally have the disadvantage that with the rigid turbine geometry they simultaneously must fulfill all requirements regarding the level of the charge air pressure to be set and the response at the various operating points of the internal combustion engine defined by rotational speed and load. Currently this is only possible by a compromise between the charge air pressure to be set and the response.
In contrast to exhaust-gas turbochargers that have a bypass valve, in exhaust-gas turbochargers featuring a variable turbine geometry, the problem lies in the optimal triggering of the variable turbine geometry. This is more complex than in the case of triggering a bypass valve and requires a new approach so as to utilize the optimal performance potential of the exhaust-gas turbocharger equipped with a variable turbine geometry.